Understanding synchronous belt drives

Many components of a synchronous power transmission belt interact with each other and combine to produce its performance characteristics. Obvious features, such as reinforcement cord and a variety of elastomeric materials, are very important to the belt carcass. But a design variation often over-looked is a belt's tooth profile, and a closer look at this critical belt feature is warranted.

Many components of a synchronous power transmission belt interact with each other and combine to produce its performance characteristics. Obvious features, such as reinforcement cord and a variety of elastomeric materials, are very important to the belt carcass. But a design variation often over-looked is a belt's tooth profile, and a closer look at this critical belt feature is warranted.

In the power transmission industry, there is no government agency that dictates conformity to standards and regulations. However, there are several industrial organizations that do write suggested and voluntary guidelines, including the Society of Automotive Engineers (SAE) and the Rubber Manufacturers Association (RMA). RMA's definitions and standards are used for this article to ensure unified terminology.

Basics

Any power transmission system utilizing synchronous belts or positive drive belts is a precision gear train. Sprockets intended to drive power or be driven by the energy in a system are given a precise number of profiled teeth evenly spaced around the periphery of a sprocket.

The pitch circle of a belt sprocket is always just beyond the outside diameter of the sprocket, which is in contrast with a chain drive sprocket having its pitch circle always within the sprocket outside diameter and one roller diameter above the tooth root diameter.

A synchronous belt is molded with teeth that are designed to mesh with the sprocket teeth. Upon close inspection, a straight belt section appears like a gear rack. As the belt engages with a sprocket, it assumes the shape of a circular gear with teeth pointing inward and located just within the pitch circle of its mating sprocket. As the belt and sprocket come into mesh, the two mating tooth profiles align, resulting in a smooth delivery of power and angular velocity (Fig. 1).

Most designs of current belt and sprocket profiles tend toward an involute gear tooth profile. This de-sign was chosen because it simplifies manufacture of the sprockets and belt molds and promotes engagement and disengagement of the meshing teeth with a minimum of interference and associated loss of efficiency due to friction.

Tooth profiles

There are visual differences between currently popular tooth profiles (Fig. 2). The trapezoidal profile design, used on synchronous belt systems as early as 1950, is still used today for generally low-tech applications where precise registration of the system is not a prime criterion. The profile is defined by RMA specification IP-24. There are metric versions called T or AT that are sometimes encountered on European machine tools.

Modern tooth profiles are generally categorized as curvilinear. These profiles are defined by RMA specification IP-27 and are designated as H profile, R profile and S profile, respectively. These designs offer improved registration or drive accuracy, ratcheting resistance, and higher power density, measured in load-per-inch-width, than original trapezoidal designs. Belt manufacturers may specialize in one design or may offer several profiles and should be consulted for specific performance claims or features of their products.

If a synchronous belt system is described as a precision gear train, the degree of precision of each element in the system needs to be considered. The precision of the metal sprockets is generally fixed by manufacturing controls, although some dimensional changes can be attributed to thermal expansion as the drive operates.

Tensioning

The elastomeric nature of the belt creates special considerations for meshing accuracy. Although constructed of high modulus, low-stretch materials, a newly installed belt starts in a relaxed condition. A belt will not properly engage with a sprocket beyond about 90° of wrap without preload. To achieve proper meshing between belt and sprocket, precise adherence to tensioning recommendations must be followed.

Proper tension provides the correct preload to bring the new belt into proper mesh with the sprockets. After the preload is correctly reapplied, dimensions of second and third generation synchronous belts become remarkably stable.

A matching tooth pitch and profile promotes smooth engagement and disengagement between belt and sprocket. The belt materials are selected to deliver strength and stability over the intended operating range while safely carrying the design load. Tensioning recommendations are determined to maintain the meshing relationship between belt and sprocket.

Matching

Matching components and following prescribed installation criteria result in a satisfactory operating synchronous belt drive system that will do the job in the most efficient, economical manner possible.

But sometimes problems develop when the reality of an application leads to compromises. Obviously, no one seeks compromises that could affect basic safety. But if a decision is based purely on economics, there may be room for design variations.

Where can compromise be made regarding belt profile, and what might be the expected outcome? Obviously, tooth pitch must be matched. A 14-mm pitch belt cannot be successfully operated in an 8-mm pitch sprocket, any more than a No. 60 pitch roller chain can run in a sprocket designed for No. 35 chain.

Also, it's equally clear that a thick-toothed belt cannot run in sprockets machined for a narrow tooth. The belt will not fit or, conversely, the resulting frictional interference would cause an unacceptable temperature rise that would transfer to all elements of the drive system.

Generally, a synchronous belt system is used be-cause of its positive drive characteristics, but there is a subtle, yet critical, difference between simply eliminating slip and maintaining precision registration between shafts. For example, elimination of slip might be seen as a technique to conserve energy, while maintaining precise registration would promote machine accuracy.

Suitable profiles

If precision registration is required, selecting the tooth profile best suited for the job is imperative (Table 1). The integrity and precision of a system can be maintained by staying with an absolute profile match between components. No substitutions should be allowed. The same holds true for drives that operate at very high speeds.

Information is needed about the rated capacity of a current synchronous belt system and the load required to transmit it. If the torque or horsepower being transmitted is 80% or more of the belt system's catalogue rating, profiles should not be mixed. This is especially true if drive failure will create a safety issue or serious financial burden. Again, stick with an absolute profile match between components.

When looking for possible profile crossovers (Table 2), there are not a lot of possibilities. Most belt or sprocket manufacturers would state that there are no crossover choices, but then, they are not facing your particular situation.

There are possible compromises between the H and R profiles. Some manufacturers offer tooth pro-files that do not strictly conform to standard configurations. Complete dimensional information is not readily available on these products. When a certain amount of experimentation is required, safety is paramount. Consult with manufacturers that may offer products designed for a universal fit. Make an enlightened compromise, but be aware of potential penalties to service life.

In addition to shortened belt life, there are other performance characteristics that could be affected, such as increased noise and vibration. Noise primarily impacts personnel close to operations, but vibration may compromise product quality, including the surface finish of a machined part. Excessive vibration may also affect shaft bearing life.

Worn sprockets

Badly worn and eroded metal sprockets present a subset of the incompatible fit condition (Fig. 3). A new belt running in a badly worn sprocket, even if its original condition provided a direct match to a belt, would compromise belt service life as quickly and surely as most other misfit conditions.

Sprockets should be considered a consumable item. As a general rule, a second belt can safely run on an original sprocket system. But, before another belt is installed, and maybe sooner when severe operating conditions exist, sprockets should be carefully inspected for wear.

If erosion is visible, especially if a step or pocket in the driving flank can be detected, replace the sprockets before installing the new belt. If this preventive step is not taken, the majority of a new belt investment will have been wasted.